the state has recorded nearly 900 cases-- that's 45% of all west nile cases in the united states. aerial pesticide spraying last month in and around dallas dramatically reduced the mosquito population. .n july, half of the mosquitos tested positive for west nile. now it's just 5%. but doctors believe the number l reported cases will increase acause weeks can pass before gople infected with west nile develop serious symptoms. dr. beth bell of the centers for disease control and prevention expects the outbreak to last rntil october. >> it takes a little time for people after they get sick to go to the doctor to get the blood test. and what we're counting are cases of illness which occurred several weeks ago. >> reporter: scott, the c.d.c. estimates that thousands more people have suffered some degree of illness but most of them never went to a doctor. >> pelley: thanks, anna. we got another important medical study today on the dangers of pro-football as the n.f.l. kicks off its new season tonight. this study, published today in ted t medical journal neurology" looks at what a career